The invention relates to a control system for a hydraulic lift driven by a variable displacement pump.
In a type of control system customarily used in applications of this nature, control means are provided which system in one of its control configurations controls a blocking valve or blocking device disposed in the consuming device line leading to the power lift, in another of its control configurations controls a hydraulic unit which switches the variable displacement pump feeding the hydraulic system to various operating states, in yet another of its control configurations regulates (via variable opening areas) a pumped flow between a pump line and the consuming device line leading to the power lift, and in still another of its control configurations regulates (via variable opening areas) a tank flow between the consuming device line and a tank line leading to a reservoir.
A control system of this customary type was necessary for a hydraulic system supplied by a constant pump because in such hydraulic systems the constant pump is shut off when the servo valve is in the neutral position, and the blocking device must be closed in order to avoid substantial power losses. This control system is also used for hydraulic systems which employ variable displacement pumps in order to reduce power losses. Such pumps are increasingly employed for this power conservation reason.
The constraint requiring the combining of switching functions and volumetric flow regulation functions by the servo valve results in disadvantages, in as much as the dynamics and accuracy of the control system are thereby limited. Inevitably the response quality of the control suffers, because in the connection to the consuming device (namely, in the consuming device line leading to the power lift) a pressure increase from a low value in the line can be accomplished only gradually.